The rally comes at a precarious time for Prime Minister Najib Razak's government as he is expected to call elections soon.

Last July the group Bersih 2.0 rallied tens of thousands in protest for electoral reform which police broke up with tear gas and water cannon, arresting 1,600 people.

"We are expecting at least double the size of last year's crowd to turn up at Dataran Merdeka on Saturday for the sit-in to demand these reforms regardless of any venue ban," organising committee member Wong Chin Huat told AFP.

City authorities have banned Saturday's sit-in at Kuala Lumpur's Dataran Merdeka – or Independence Square – fencing off the downtown venue from early Friday to Sunday.

Police also served a court order on protest organisers Bersih 2.0. The order bans the public from gathering near the square and warns people to stay away.

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"Any gathering at Dataran Merdeka and its adjoining land is prohibited, and the public is given warning not to turn up, attend or take part in any gathering from April 28 till May 1," it said.

The police crackdown on the electoral reform rally organised by the group last year drew national and international condemnation.

Following that, Mr Najib set up a bipartisan parliamentary panel which suggested several changes to the electoral system but Bersih said they fell short of its demands.

Bersih 2.0 chairwoman Ambiga Sreenivasan insisted on Friday the gathering would go ahead at the square and accused the city's mayor of abusing his power by banning the rally.

Bersih has rejected an offer to hold the protest in alternative venues, including stadiums, saying it is too late to change the location.

"We will march to the barrier. We will not break the barriers or create any situation in the hopes that the government will lift the barrier and allow the sit-in to take place," she told a news conference.

Although polls are only due next year, Mr Najib is expected to seek a fresh mandate soon.

The ruling Barisan Nasional coalition has governed Malaysia for more than five decades but a resurgent opposition made unprecedented inroads in the last election in 2008.

Ibrahim Suffian, director of independent pollster Merdeka Centre, said Najib should have embraced the protest – in line with his reform agenda to allow for greater civil liberties – rather than trying to prevent it.

"If he cracks down he will lose the goodwill he has created over the past four years," he told AFP.

Foreign Minister Anifah Aman, however, urged people to "respect the laws of this country" and refrain from gathering at the square.

"The government has no interest to deny Bersih's right to peaceful protest," he told reporters. "However, I want to urge people to be reasonable."

The Bangkok-based Asian Forum for Human Rights and Development, representing 49 human rights groups across Asia, urged Malaysia "to respect the fundamental right to peaceful assembly" by allowing the rally to proceed.

500 people are expected to march in London on Saturday morning from the Malaysian High Commission to the British Parliament, where they will stage a sit-in.